Convert memo == '' to null to make querying cleaner (#105)

Author: mxr

sqlite_export_for_ynab/ddl/create-relations.sql

@@ -171,7 +171,7 @@ SELECT
             COALESCE(st.transfer_account_id, t.transfer_account_id) IS NULL
             THEN COALESCE(st.category_name, t.category_name)
     END AS category_name
-    , COALESCE(st.memo, t.memo) AS memo
+    , COALESCE(NULLIF(st.memo, ''), NULLIF(t.memo, '')) AS memo
     , COALESCE(st.payee_id, t.payee_id) AS payee_id
     , COALESCE(st.payee_name, t.payee_name) AS payee_name
     , COALESCE(st.transfer_account_id, t.transfer_account_id)
@@ -261,7 +261,7 @@ SELECT
             COALESCE(st.transfer_account_id, t.transfer_account_id) IS NULL
             THEN COALESCE(st.category_name, t.category_name)
     END AS category_name
-    , COALESCE(st.memo, t.memo) AS memo
+    , COALESCE(NULLIF(st.memo, ''), NULLIF(t.memo, '')) AS memo
     , COALESCE(st.payee_id, t.payee_id) AS payee_id
     , COALESCE(st.transfer_account_id, t.transfer_account_id)
         AS transfer_account_id